The present invention is related to an improved solid electrolytic capacitor and an improved method of manufacturing a solid electrolytic capacitor. More specifically, the present invention is related to an improved treatment method which provides for improvements in the coating quality and physical properties of the finished capacitor.
Solid electrolytic capacitors have been widely used for many years throughout the industry. Of particular interest herein is a solid electrolytic capacitor comprising a cathode of an intrinsically conducting polymer such as polyaniline, polythiophene or polypyrroles. The polymers are usually either formed in-situ or by dipping into a slurry of polymer. With in-situ formation a capacitor precursor is introduced into a monomer solution wherein the monomer is polymerized either electrochemically or by chemical means.
It is well known in the art that when forming a coating from polymer slurry achieving an adequate coating on the edges and corners is very difficult. The slurry tends to pull away from the edges and corners, believed to be due to surface tension driven capillary effects. The result is thin layers and, in some cases, voids or holes within the coating, which is detrimental to achieving a high quality capacitor. One of skill in the art typically applies multiple layers, beyond those needed for capacitance, to insure adequate coverage of the edges and corners.
Efforts to mitigate edge and corner thinning are described throughout the literature as exemplified in U.S. Pat. Publ. No. 2012/0057275 wherein cross-linkers comprising at least one diamine, triamine, oligamine or polymeric amine is applied before the application of the polymer slurry. While advantageous, the use of strong ionic acid anions with the cross-linking technology has now been understood to be detrimental due to corrosion of the underlying anode. This is a particular problem with an aluminum anode wherein the corrosion occurs rapidly.
Yet another problem with the cross-linkers is that each slurry layer tends to dry and form a skin. Subsequent layers then do not adequately migrate through the skin into previous layers and the layers are joined by adhesion not cohesion. The result is the formation of a cathode wherein discrete layers separate, or delaminate, under harsh conditions thereby decreasing conductivity between adjacent layers and increasing equivalent series resistance (ESR). The stability and reliability of capacitors made with the layered coating of conductive polymer is also undesirable.
In spite of the advances made in the art there is still a significant need for capacitors which maintain their properties in adverse conditions without loss of capacitance due to anode corrosion or an increase in ESR due to cathode degradation and layer separation.